This invention relates generally to radio transceiver apparatus and more particularly to in-band, radio relay type transceiver apparatus.
In the deployment of a remote sensor-data collection system it is frequently necessary to include a number of radio repeater sites to overcome the line of sight transmission restrictions, thereby extending the range of data collection. The fundamental problem, however, in implementing any radio relay is in the preventing of the relatively high power signal from the relay's transmitter from being picked up by its own receiver at a level high enough to desensitize it. Prior art radio relays have utilized a variety of techniques to achieve the required transmitter to receiver isolation, for example, a conventional out-of-band relay is implemented with the transmitter frequency f.sub.t and receiver frequency f.sub.r being in different bands and with isolation being achieved by means of fixed f.sub.t and f.sub.r band filters. The problems associated with such an apparatus, however, is the frequency allocation problem which presently has many undesirable restrictions. With respect to the conventional in-band relay system, a relatively large f.sub.t and f.sub.r frequency separation exists between the operational transmit and receive frequencies. However in such apparatus tunable narrow band filters are utilized which become relatively large and expensive and many frequency channels are inherently unusable. Another type of radio relay system known to those skilled in the art is the store and forward relay type of system which alternately receives and transmits each message. The problem associated with this type of system is that the relays require a data storage capability which encounters severe problems where analog data is being stored and also only a 50% receiver duty cycle is possible. Another known type of relay system comprises what is known as a sampling relay wherein the message is alternately received and transmitted at a Nyquist sampling rate. Such a system has an inherent limitation that it is subject to spectrum splatter and loss of isolation in ground environments due to delayed reflections. All of the above noted radio relays typically include a common antenna which is shared by both the transmitter and receiver portions of the relay. Also known are radio relay systems which operate at any transmit and receive frequency but these systems include respective directional antennas which are highly directive. In such systems no omnidirectional coverage is obtainable and the antennas themselves tend to become relatively large at VHF frequencies.
Accordingly, it is an object of the present invention to provide an improvement in radio relay apparatus.
It is another object of the present invention to provide an in-band radio relay apparatus which includes improved isolation between transmitter and receiver.
It is yet another object of the present invention to provide an in-band radio relay apparatus which includes adaptive coherent interference cancellation to prevent the transmitter from desensitizing the receiver during operation.